Microcapsule technology has been in existence for a number of years. Microcapsules have a variety of uses, especially for containing dyes, inks, chemical reagents, pharmaceuticals, flavoring materials, and more especially agrochemicals, that is fungicides, bactericides, insecticides, herbicides and the like.
The development and uses of microencapsulation are described by Gordon Marrs and Herbert B. Scher in Chapter 4 of "Controlled Delivery of Crop Protection Agents" (London, Taylor and Francis, 1990). As discussed by Marrs and Scher, there are three methods of forming microcapsules: i) physical methods, ii) phase separation methods and iii) interfacial polymerization.
In the third of these methods, the walls of microcapsules are generally formed of polymeric material produced by a polymerization reaction which preferably takes place at the interface between two phases, usually an aqueous phase and a water-immiscible organic phase. Thus, they may be produced from a water-in-oil emulsion or more usually an oil-in-water emulsion.
A basic patent dealing with microcapsule technology is U.S. Pat. No. 4,285,720. In this patent the walls of the microcapsules are produced from polymers formed by reactions of isocyanate monomers.
A second means of forming microcapsules by interfacial polymerization is described in U.S. Pat. No. 4,956,129. In this patent polymeric microcapsule walls are produced from etherified urea-formaldehyde prepolymers which undergo self-condensation polymerization under acid conditions.
Various improvements on these techniques have been suggested. For example, U.S. Pat. No. 4,140,516, describes the use of phase transfer catalysts while U.S. Pat. No. 4,448,929, describes the use of an improved protective colloid. However, in all these patents, the process have been applied only to liquids, i.e., to materials which are liquid at ambient temperature or to solutions. Unfortunately, many biologically active compounds are solids with high melting points and are not readily soluble in most commonly used solvents. The benefits of microencapsulation e.g., controlled release and increased longevity of efficacy have not been readily available to such compounds using known techniques.
It is also known to surround solids by a polymer matrix. Thus, in U.S. Pat. No. 4,428,983, there is described a process for producing quartz crystals in a polymer matrix. The patent uses the term suspension for describing the paste of quartz crystals in the prepolymer, but this publication does not describe the production of microcapsules containing a solid suspended in a liquid.
There are a large number of publications dealing with the production and application of microencapsulated formulations of haloacetanilide herbicides. These include U.S. Pat. No. 4,280,833; 4,417,916; 4,534,783; 4,563,212; and 4,640,709. Additionally, U.S. Pat. No. 4,936,901 discloses herbicidal compositions which are dry flowable water-dispersible granular formulations comprising a mixture of microcapsules of a water-insoluble pesticide (including a haloacetanilide herbicide) encapsulated within a polymeric shell wall and at least one other pesticide which is nonencapsulated. Such compositions were necessary since no satisfactory techniques to produce a microcapsule containing a solid, biologically activate herbicide suspended in a liquid have been known.
It is not surprising that capsules containing a biologically active solid suspended in a liquid have not been made up until the present time since the problems to be faced in producing such a capsule are formidable. For example, in forming such capsules from an oil-in-water emulsion, the following difficulties must be addressed:
Firstly, a stable suspension of the solid in a water-immiscible liquid must be produced. If dispersants or surfactants are used, they must not interfere with further processes of dispersion used in making microcapsules.
Secondly, the suspension must be dispersed in water to produce stable, well dispersed droplets. For biologically active substances, it is preferable to have very small droplets of liquid dispersed in water to present a high surface area in the resulting microcapsules. To produce very small droplets requires high shear forces which would tend to break down the droplets and/or release the solid from suspension. Surfactants are usually required to achieve good dispersion and stable droplets.
Thirdly, the presence of one or more surfactants can make the dispersed droplet system unstable and the phenomenon of phase inversion may occur i.e., the water forms small droplets within the liquid, a water-in-oil emulsion.
Fourthly, the solid suspended in the water-immiscible liquid is liable to migrate to the aqueous phase, particularly when emulsifying surfactants are used.